This invention relates to a method for establishing a telecommunication connection from a subscriber station of a radio system forming a wireless subscriber connection to a public switched telephone network (PSTN), in which method a call request is received from the subscriber station. The invention further relates to a radio system forming a wireless subscriber connection, comprising subscriber stations and a subscriber network element comprising means for transmitting data signals between subscriber stations and an exchange of a PSTN network, the system comprising at least one subscriber station comprising means for sending a call request to a subscriber network element by means of radio-frequency signals when the user interface of the subscriber station has shifted into the OFF-HOOK state.
The present invention is related to a WLL system (Wireless Local Loop), that is, to a radio system providing a wireless local loop, in which system the subscriber stations are connected by a radio connection via base stations and a WLL subscriber network element to a PSTN exchange. It is possible to connect to the WLL system any conventional type of user interface, that is, a telephone set, the radio path thus being transparent to the user.
A subscriber station refers herein to the equipment the user has available for transmitting and receiving data signals, i.e. in the case of a WLL subscriber station, the subscriber station comprises a terminal equipment consisting of a radio part and a teleadapter, as well as a user interface to be connected to the terminal equipment, such as a telephone set, telefax, computer-modem combination or the like.
Prior art WLL systems are usually based on some cellular radio system, whereby an exchange of the cellular radio system used as the basis of the system functions as the WLL subscriber network element, with the exception that unnecessary features have been removed from the exchange. The signalling of the WLL system thus corresponds to the cellular radio system on which the WLL system is based as much as possible, and mobile stations of the cellular radio system in question may thus also be used.
A WLL system is previously known in which a signal generator is integrated to a WLL subscriber station for producing sound signals to be fed to the user interface in connection with call setup. In other words, when the user interface shifts into the OFF-HOOK state, the signal generator of the WLL terminal equipment feeds either a busy-tone (if the radio connection is poor), or dialling-tone to the user interface. In this prior art system, once the user has fed the dialling desired by means of his user interface, the subscriber station starts call setup after the duration of six seconds from the last digit that has been dialled. In other words, since the subscriber station cannot analyse the series of digits fed from the user interface, the above-mentioned time supervision of six seconds is necessary for the subscriber station for concluding when the dialling is complete. When the time-supervision of six seconds expires, i.e. the subscriber station has not received any dialling during the last six seconds, the subscriber station sends the dialling it has buffered in form of digit frames to the subscriber network element. The subscriber network element converts the received frames into tone-diallings by means of an MFT converter (Multi-Frequency Tone) and forwards them to the exchange. Once the subscriber network element has sent the diallings to the exchange, it sends an acknowledgement to the subscriber station and connects a speech path between the exchange and the subscriber station.
The most remarkable weakness of the system described above is that the user of the subscriber station does not hear the sound signals sent from the exchange. For instance, in a situation in which the user of the subscriber station has previously programmed a call transfer by means of the user interface of his subscriber station and unintentionally forgotten to cancel the call transfer, a sound signal different from the normal dialling tone is usually transmitted from the PSTN exchange when a new call is being established, the purpose of said sound signal being to indicate to the user that the call transfer is on. Since the sound signals fed by the exchange are not forwarded to the subscriber during call set-up in the above-described WLL system, the user of the subscriber station in question does not hear the sound signal intended for him at any stage.